Previous work in our laboratory revealed the importance of a family of protein kinases, MUK/MLK family, as activators of JNK signaling cascade. In the present study, we first clearly shown that these protein kinases are the kinases that directly phosphorylates and activates the JNK activators, MKK7 and SEK1. Furthermore, we found that the mode of the activation of the JNK pathway through this family of protein kinases is unique in that the steady state levels of mRNAs for MUK and MST are regulated under cellular stress conditions. This raises the possibility that the activation of JNK pathway might involve the regulation of the amount of mRNA and/or protein of MUK/MST family protein kinases. In addition to such unique regulation, there is a protein, MIPP, which directly binds to MUK and destabilizes MUK.Further, there is a protein, 3-3, which binds to MUK and activates its ability to activate JNK pathway. Further studies of the mode of activation of JNK through MUK/MLK family of protein kinases is required for clarification of the physiological importance of this unique family of protein kinases in cellular stress responses.